Planets have a number of important physical characteristics which affect their usefulness to human beings. Most of these properties can be measured from a ship within the star system. The ship's computer will summarize the data collected by the probes, and present it in tabular form, along with a synthesized habitability index, which represents the computer's judgement as to the suitability of the planet. A rating of 100 is the highest possible, and most earth-like planets have ratings between 90 and 95.
Planets have a similar rating to habitability, it is an index of either 0 or 15 to 100. This index is an indication how much ore can be mined per miner per year.
Gas giants can have atmospheres rich in hydrogen, ships are equipped with a scoop allowing for an almost unlimited range.
The ships computer will indicate this with a fuel rating, the higher the rating the more hydrogen is collected per scoop.